Blautopf, Natural spring in Blaubeuren, Germany.
Blautopf is a karst spring in Blaubeuren that forms a funnel-shaped basin and marks the origin of the river Blau. The water rises from an underground cave system and reaches a depth of about 21 meters (69 feet) in the center.
The spring served as a landmark in the region since medieval times and early attracted the interest of researchers. In the 20th century divers explored the branching cave system behind it and mapped several kilometers of underground passages.
The name comes from the intense blue color that the water shows under certain light conditions and has long stirred the imagination of local people. Today walkers and families come here to sit by the basin edge and watch the play of colors.
The spring lies at 512 metres (1,680 feet) above sea level on the northern edge of Blaubeuren, about 16 kilometres (10 miles) west of Ulm. It can be reached by a short detour from the Danube Cycle Path or on foot from the town centre. The water level rises and falls sharply with rainfall, from 250 to 32,670 litres per second.
The water changes its blue intensity depending on light conditions and limestone content, giving the basin an almost artificial glow. On sunny days the surface looks like polished turquoise, while on overcast days it shifts into a deep gray.
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